Test depending on core.async thread names will likely break soon

This test code [core-async-dispatch-thread?](https://github.com/clojure-lsp/lsp4clj/blob/master/test/lsp4clj/server_test.clj#L484-L485) depends on the name of core.async threads, which is going to change soon and is not considered part of the API, so fyi, it will likely stop working.

Start with test/lsp4clj/server_test.clj around lines 484-485 and inspect core-async-dispatch-thread?. Determine how the test relies on core.async thread names and identify a stable way for the test to verify dispatch behavior. Done means the test no longer depends on names that are outside the API and still passes.

Written by the indexing model from the issue text.